This invention relates to a system for amplitude modulating a radio frequency carrier and, more particularly, for generating a square wave pulse train, the pulse widths of which vary as a function of the amplitude of the arc sine-converted input audio signal.
Amplitude modulation of a carrier wave can be achieved by generating a rectangular wave pulse train, the pulse widths of which vary as a function of the audio frequency input, and then passing the pulse train through an appropriate band pass filter.
A system for amplitude modulating a high frequency carrier wave is described in U.S. Pat. No. 3,363,199 issued Jan. 9, 1968 entitled "Device for Amplitude Modulating a High Frequency Carrier Wave." The system disclosed therein achieves modulation by generating a rectangular waveform at the carrier frequency wherein the relative pulse width .phi. is determined approximately by the relationship: EQU .phi. = 2 arcsin (1 + a)/2
wherein a is the instantaneous amplitude of the modulating waveform divided by the value at full modulation.
A sinusoidal waveform at the carrier frequency is in one embodiment of the invention described in U.S. Pat. No. 3,363,199 fullwave rectified and then subtracted from the modulating waveform to which a D.C. voltage having a magnitude of three times the maximum amplitude of the modulating waveform has been added. A flip-flop circuit then generates positive and negative going pulse trains which after amplification are recombined and filtered so as to generate the amplitude modulated carrier. In an alternative embodiment of this patent, the sinusoidal waveform at the carrier frequency is half wave rectified so as to separate the positive and negative half waves of the high frequency sinusoid. The positive and negative half waves are then operated upon so as to generate approximately the desired amplitude modulated carrier.
This prior art arrangement in addition to being complex by operating on a carrier wave to produce the above described relationship also introduces distortions since the carrier signal is rectified and the modulated signal is an unprocessed audio signal.